Subpial glial limiting membrane of the cat spinal cord visualized by scanning electron microscopy

The subpial glial limiting membrane of the cat spinal cord was examined by scanning electron microscopy after disruption of plasma membranes of astrocytes composing this membrane by controlling the fixation time in glutaraldehyde and OsO4. The subpial glial limiting membrane of the spinal cord was principally composed of two layers of astrocytic processes: an upper layer mainly composed of laminated flattened processes and a lower layer mainly composed of thin, cord-like processes. The outermost surface of the limiting membrane immediately beneath the subpial basement membrane was totally lined with flattened, overlapping astrocytic processes. The majority of the flattened processes in the upper layer of the limiting membrane were varied in shape and usually smaller than 5 x 5 microns2 and the rest of the flattened processes were oval or round, and larger (5-10 microns in diameter) than the majority of the flattened processes. Glomerular structures of tightly packed glial filaments were observed within the superficial flattened processes. These glomerular structures appeared to occupy the central space as a cytoskeletal core. The thin, cord-like processes contained a varying number of glial filaments running parallel to one another to form narrow bundles of filaments. These bundles of filaments coursed obliquely toward the outer surface of the spinal cord where they were transformed into superficial flattened glomerular structures of various forms.